The invention concerns a method of handling fuel assemblies and rods in the reloading with fuel of a nuclear reactor containing assemblies with a framework closed by two caps inside which fuel rods are vertically disposed.
After a certain working period, fuel assemblies disposed in the vessel of the reactor exhibit some wear, i.e. a nuclear fissionable fuel material of these assemblies is depleted to some extent, so that it is necessary to change some of these fuel assemblies which have become unsuitable for later use in the core of the nuclear reactor for the production of heat. In addition, the cladding material of some fuel rods can exhibit cracks after these rods have been used for some time, so that leakages of fission products into the cooling fluid of the reactor, in contact with these fuel rods, are possible.
The core of the nuclear reactor is consequently reloaded with fuel at predetermined time intervals, during which used or defective fuel assemblies are removed and replaced.
In most nuclear reactors, the fuel elements constituted by rods containing fuel material inside a tubular cladding of cladding material are disposed inside assemblies in which these rods are placed parallel to each other, in the axial direction of the core, i.e., generally in the vertical direction. Each of the fuel-rod assemblies constitutes a rigid assembly having a framework or skeleton inside which the fuel rods are disposed. The framework of the assembly is constituted by support tubes and caps which can be designed to allow de-mounting and remounting of the assembly if the framework is required to be reused when the fuel element of the fuel rods is used up.
In the case of pressurized water nuclear reactors, the assemblies are disposed in the tank of the reactor and constitute its core. The assemblies are disposed in a lattice of square mesh, and some of these assemblies are equipped with a cluster of material absorbing the neutrons serving to control the reactor. During reloading of the reactor, the vessel of the nuclear reactor is open, and this vessel as well as the swimming pool around it is filled with water up to a certain level, allowing fuel rods to be moved between the reactor and the swimming pool, at a depth sufficient to assure the protection of the personnel carrying out the reloading operations.
These operations consist firstly in moving fuel assemblies from one region of the reactor core to another, this core being divided into three regions inside which each of the fuel assemblies lies between two reloading operations. The most used assemblies are to be found in the third region of the reactor and are removed via the swimming pool for the fuel adjoining the reactor and replaced by new assemblies.
When transferring assemblies from one position to another or replacing assemblies by new assemblies, it is necessary to carry out permutations of the control clusters, these clusters always staying in the same position in the core.
The reloading operations also consist in replacing fuel assemblies having rods exhibiting leakages by new assemblies.
Independently of the reloading operations, the used assemblies are removed after decay of their radio activity to reprocessing works, although a certain number of fuel rods inside these assemblies could be reutilized to reconstitute reloadable fuel assemblies in the nuclear reactor.
Similarly, part of the fuel rods of the assemblies removed from the core of the nuclear reactor because they contain fuel rods exhibiting leakages or geometric deformations can be recovered to reconstitute reloadable assemblies in the core of the reactor.
Methods of repairing fuel assemblies by replacing used fuel rods inside the assembly have also been proposed.
In these methods, however, repair of the assembly is carried out, independently of the reloading operation, with a tool allowing the defective fuel rods to be removed one by one and replaced by new rods.
Reloading is always carried out with whole assemblies, each used or defective assembly being replaced by a new or reconstituted assembly reconditioned in operations which are independent of the actual reloading operations. Known methods generally require reprocessing and storage of the whole of the assembly comprising both skeleton and fuel rods.
The methods used to date consequently tend to increase the consumption of fuel material by removing fuel rods which could continue to be used, and to increase the number of de-mounting and handling operations carried out on fuel assemblies to recondition them before loading into the core.